Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called Iub interface. The second generation wireless communication system (2G), also known as GSM, is a well-established cellular, wireless communications technology whereby “base transceiver stations” (equivalent to the Node B's of the 3G system) and “mobile stations” (user equipment) can transmit and receive voice and packet data. Several base transceiver stations are controlled by a Base Station Controller (BSC), equivalent to the RNC of 3G systems.
Communications systems and networks are developing towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network, and a System Architecture Evolution (SAE) solution, namely, an Evolved Packet Core (EPC), for a mobile core network. An all IP EPC and an E- UTRAN together are often referred to as an Evolved Packet System (EPS). An EPS provides only packet switching (PS) domain data access so voice services are provided by a 2G or 3G Radio Access Network (RAN) and circuit switched (CS) domain network or Voice over IP (VoIP) techniques. User Equipment(UE) can access a CS domain core network through a 2G/3GRAN such as the (Enhanced Data Rate for GSM Evolution, EDGE) Radio Access Network (GERAN) or a Universal Mobile Telecommunication System Terrestrial Radio Access Network (UTRAN), and access the EPC through the E-UTRAN.
Some User Equipments have the capability to communicate with networks of differing radio access technologies. For example, a user equipment may be capable of operating within a UTRAN and within an E-UTRAN.
Lower power (and therefore smaller coverage area) cells are a recent development within the field of wireless cellular communication systems. Such small cells are effectively communication coverage areas supported by low power base stations. The terms “picocell” and “femtocell” are often used to mean a cell with a small coverage area, with the term femtocell being more commonly used with reference to residential small cells.
Small cells are often deployed with minimum RF (radio frequency) planning and those operating in consumers' homes are often installed in an ad hoc fashion. The low power base stations which support small cells are referred to as Access Points (AP's) with the term Home Node B (HNB's) or Evolved Home Node B (HeNB) identifying femtocell Access Points. Each small-cell is supported by a single Access Point. These small cells are intended to augment the wide area macro network and support communications to multiple User Equipment devices in a restricted, for example, indoor environment. An additional benefit of small cells is that they can offload traffic from the macro network, thereby freeing up valuable macro network resources An HNB is an Access Point that provides a wireless interface for user equipment connectivity. It provides a radio access network connectivity to a user equipment (UE) using the so-called Iuh interface to a network Access Controller, also known as a Home Node B Gateway (HNB-GW). One Access Controller (AC) can provide network connectivity of several HNB's to a core network.
Typical applications for such Access Points include, by way of example, residential and commercial locations, communication ‘hotspots’, etc., whereby Access Points can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, small cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion or poor coverage at the macro-cell level may be problematic.
Thus, an AP is a scalable, multi-channel, two-way communication device that may be provided within, say, residential and commercial (e.g. office) locations, ‘hotspots’ etc, to extend or improve upon network coverage within those locations. Although there are no standard criteria for the functional components of an AP, an example of a typical AP for use within a 3GPP 3G system may comprise Node-B functionality and some aspects of Radio Network Controller (RNC) functionality as specified in 3GPP TS 25.467. These small cells are intended to be able to be deployed alongside the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment.
Herein, the term “small cell” means any cell having a small coverage area and includes “picocells” and “femtocells.”
It is often useful in cellular networks (including small cell networks) to obtain the permanent subscriber identity (IMSI) of the UEs which are accessing the small cells. This identity can be used for various purposes, one being to track UEs as they move through the macrocell or small cell network. In a 3G system, a common technique in small cells is for a Home Node B, (an Access Stratum device), to request the UE's permanent subscriber identity (IMSI) using an NAS (Non-Access Stratum) Identity Request message, normally used by the Core Network. The IMSI which results can be used for access control within a cell or for logging particular performance behaviours of that UE. In LTE on the other hand, the protocol design authenticates and optionally encrypts the Non-Access Stratum (NAS) between the UE and the MME (Mobility Management Entity) using separate keys from those used for authentication and typically encryption in the Access Stratum or the RRC (Radio Resource Control) layer, which is authenticated and typically encrypted between the UE and the eNode B and these NAS keys are not known to the Access Stratum devices such as the eNode B. Thus, the NAS security deployed in LTE means that an eNode B (or a Home eNode B) cannot use the same technique as is used in 3G, specifically cannot send a valid NAS Identity Request message to the UE, as it cannot generate the valid integrity protection code used for authentication of the message by the UE. Such a request would have to be issued by the MME and even if the MME were to issue such request, the UE response would not be readable by the eNode B if NAS encryption were enabled. There is no standard method for the eNode B to request that the MME generates such request and the MME will only rarely generate such a request under normal operation.